Two factors determine what type of rock forms. The composition of the magma determines if the rock is mafic, felsic, or intermediate. The rate the magma cools determines the texture of the rock.

Minerals can form from the cooling of magma. Density differences can force magma upward into cooler layers of Earth’s interior. If magma cools slowly, atoms do not have time to arrange themselves into large crystals. Small crystals form from rapidly cooling magma.

The most common way to divide lava flows into distinct types is following: Pahoehoe lava flow, Aa lava flow, Blocky lava flow, and also Pillow lava flow. Sometimes Turbulent lava flow is also added, but the latter is only of theoretical interest to scientist because we will not see that type of lava flow in the nature.

The lavas of different volcanoes, when cooled and hardened, differ much in their appearance and composition. If a rhyolite lava-stream cools quickly, it can quickly freeze into a black glassy substance called obsidian.

Pahoehoe

In general, lava flows slowly, with typical speeds of 0.25 mph (0.40 km/h) and maximum speeds of 6 to 30 mph (9.7 to 48.3 km/h) on steep slopes.

Lavas, particularly basaltic ones, come in two primary types: pahoehoe (pronounced ‘paw-hoey-hoey”) and aa (pronounced “ah-ah”). Both names, like a number of volcanological terms, are of Hawaiian origin. A third type, pillow lava, forms during submarine eruptions.

The smooth variety is called pahoehoe, and the rougher variety is known as a’a (pronounced ah-ah). A’a is a Hawaiian word meaning “stony with rough lava”. During an eruption, a’a lava comes out of the volcano as a very thick (viscous) lava that travels very slowly. The inside of an a’a lava flow is thick and dense.

This type is called a shield volcano. Most shield volcanoes are formed of fluid basaltic lava flows. Mauna Kea and Mauna Loa are shield volcanoes. They are the world’s largest active volcanoes, rising over 9 km above the sea floor around the island of Hawaii.

characteristics of flow …the Hawaiian names pahoehoe and aa (or a’a). Pahoehoe lava flows are characterized by smooth, gently undulating, or broadly hummocky surfaces. The liquid lava flowing beneath a thin, still-plastic crust drags and wrinkles it into tapestry-like folds and rolls resembling twisted rope.

‘A’ā (pronounced “ah-ah”) is a Hawaiian term for lava flows that have a rough rubbly surface composed of broken lava blocks called clinkers. The incredibly spiny surface of a solidified ‘A’ā flow makes walking very difficult and slow.

Walking on aa lava flows is difficult and dangerous because of the sharp spiny surface. The rocks are unstable and you can easily fall when walking on them causing you to hit the sharp glassy surfaces of the clinkers.

Dormant volcanoes have not erupted for a very long time but may erupt at a future time. Extinct volcanoes are not expected to erupt in the future. Inside an active volcano is a chamber in which molten rock, called magma, collects.

How old is the oldest volcano? The oldest volcano is probably Etna and that is about 350,000 years old. Most of the active volcanoes that we know about seem to be less than 100,000 years old.

What causes volcanoes to go extinct? Simply put, they’re cut off from their supply of lava. This is where a chamber of magma underneath the surface of the Earth finds its way to the surface through weaknesses in the crust. A good example of this is the hotspot that created the chain of Hawaiian Islands.

An extinct volcano is “dead” — it hasn’t erupted in the past 10,000 years and is not expected to ever erupt again. For example, if a volcano’s eruptive history shows that it usually erupts every 10,000 years or so, and there hasn’t been an eruption for a million years, it may be called extinct.